Help with Centripetal Force question

1. The problem statement, all variables and given/known data
A 1000 kg sports car moving at 20 m/s crosses the rounded top of a hill (radius = 100m). Determine (a) the normal force on the car, (b) the normal force on the 70 kg driver, and (c) the car speed at which the normal force is zero.

To find the speed the car would have to move for the normal force to become zero, I would need the acceleration from gravity and acceleration from the centripetal force to cancel each other out:
Fn = 1000 kg * 0 m/s^2 = 0 N
a = -10 m/s^2 + 10 m/s^2